At its simplest, you become a hunter by learning about the supernatural world. For some people, its a family legacy. If hunters become parents, they'll often teach their kids. Some learn the hard way by having the supernatural thrown on them. Maybe your wife gets possessed by a demon. Maybe your best friend is killed by a vengeful ghost.

You don't need to know the overarching mythology. You can take it on a small scale.

In the old days, hunters who found out usually just had to learn the ropes for themselves and study lore. Nowadays, the Men of Letters is an organization that coordinates and instructs hunters.

If for instance, your character learned about the supernatural because her boyfriend was turned into a vampire, you could have it be that another hunter saved her from being turned by her boyfriend and killed him. Afterward, when the hunter couldn't convince her to go back to her old life, he referred her to the Men of Letters to answer her questions.

hmmmm, i was thinking of a cop that wanted to protect the people from favelas(brazil), and in one of the gang encounters he faces with something supernatural? but i doubt men of letters have a global reach, probably only inside usa? :0

Hey, wait, how great is this idea. What if your cop is going after a human trafficking ring, only it turns out that the human traffic is for rich vampires who don't like to have to go hunting for food. So my hunter going after these vamps encounters your cop, and the duo is formed. :O

He should totally come across my guy beheading someone and think my guy is a nut and tries to arrest him lol. Then my hunter shows him the fangs.

I like him. :0 My only concern is that if you have him based in a different country, he won't be able to take part in the overarching plot. But if you don't mind keeping his story small and personal, then I think it's fine. It would be kind of fun to have a separate side plot of this cop and hunter working together off on their own with their own little plot.